As a technology of detecting tampering of a paper document, there is a technology of calculating characteristic data including black pixel density and the like from a document and adding the characteristic data to a margin of the document in a format of a barcode or the like to make it possible to detect, when the document is tampered, the tampering of the document (see Patent document 1). There is also a technology of superimposing and printing dot patterns having regularity to make it possible to detect disorder of the regularity of the dot patterns due to tampering (see Patent document 2). Besides, several technologies of preventing tampering of a document are proposed (see Patent documents 3 to 8).    [Patent document 1] Japanese Laid-open Patent Publication No. 2001-309157 A    [Patent document 2] Japanese Laid-open Patent Publication No. 2005-12530 A    [Patent document 3] Japanese Laid-open Patent Publication No. 2002-109113 A    [Patent document 4] Japanese Laid-open Patent Publication No. 2003-264685 A    [Patent document 5] Japanese Laid-open Patent Publication No. 2003-209676 A    [Patent document 6] Japanese Laid-open Patent Publication No. 2006-14189 A    [Patent document 7] Japanese Laid-open Patent Publication No. 2007-28324 A    [Patent document 8] Japanese Laid-open Patent Publication No. 2007-281539 A